Handoff and Dropped Calls Strategies and Evaluation

1. CMC UnitV PRESENTATION BY VIDYA SAGAR

2. UNIT – V HANDOFFS AND DROPPED CALLS Handoffs initiation, Types of handoff, Delaying handoff, Advantages of handoff, Power difference handoff, Forced handoff, Mobile assisted and soft handoff. Intersystem handoff, Introduction to dropped call rates and their evaluation.

3. Handoff Strategies Handoff: a mobile user moves to a different cell while conversation is in progress, MSC transfers the call to a new BS. Identifying new BS New voice and control channels to be allocated Handoff must be performed Successfully Infrequently To achieve this, designer must specify optimum signal level at which handoff initiates vidya sagar

4. Need of Handoff? • Power Consideration • Traffic Consideration • Channel Quality Consideration • Distance Consideration • Administrative Consideration vidya sagar

5. Once, a signal level is specified as min usable level for acceptable voice quality A slightly stronger signal level is used as threshold Normally taken between -90 dBm and -100 dBm. This margin ∆ = Pr_handoff – Pr_min, can not be too large or too small If ∆ is too large, unnecessary handoffs, burden on MSC If ∆ is too small, insufficient time to complete a handoff before a call is lost due to weak signal ∆ should be chosen carefully to meet conflicting requirements vidya sagar

7. Signal Coverage Cells

8. Handover decision receive level BTSold receive level BTSnew HO_MARGIN MS MS BTSold BTSnew vidya sagar

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10. CASE 1: Handoff - Successful Value of delta is large enough. When the PHandoff is reached, the MSC initiates the handoff. vidya sagar

11. CASE:2 Handoff - Unsuccessful In this case, the MSC was unable to perform the handoff before the signal level dropped below the minimum usable level, and so the call was lost. vidya sagar

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13. Handoff – Unsuccessful (cont’d) • Reasons for failed handoff: • ∆ too small (i.e. PHANDOFF too low) • high mobile speeds • Excessive delay at MSC • High traffic level • Un-availability of channels vidya sagar

14. Dwell time • The length of needed monitoring largely depends on the speed of mobile units. • Dwell time: the amount of time over which a call may be maintained within a cell without handoff. The statistics of dwell time, vary greatly depending on the type of radio coverage and user profiles within a cell, are important in the practical design of handoff algorithms. vidya sagar

15. Two Decision-Making Parameters of Handoff Type -1 • the signal-strength threshold level for handoff is −100 dBm – noise-limited systems −95 dBm – interference-limited systems • Location receiver at each cell site • Received signal strength (RSS) • RSS = C + I • Two situations may occur • I more • I Less • Not accurate • Based on • signal strength • carrier-to-interference ratio

16. Type -2 C/I at the cell boundary for handoff should be at a level, 18 dB • Carrier to Interference Ratio • Two situations occur • C Less • I more

17. Relative signal strength with hysteresis and threshold vidya sagar

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19. Handoff scenarios Depending on the BS and MSC arrangement of the cellular network the handoff may occur in the following scenarios, based on the movement of a mobile station (MS). vidya sagar

20. Classification based on natures of handoff • In this classification, the handoff mechanism is usually categorized as follows: • Hard handoff : A hard handoff is also known as break-before-make handoff. • Soft handoff : The soft handoff is also known as make-before-break handoff. Soft handoff between BS1 and BS2 vidya sagar

21. Comparison of hard handoff and soft handoff vidya sagar

22. Classification based on purposes of handoff In this classification the handoff can be of three types: intra-cell handoff, inter-cell handoff, and inter-system handoff. vidya sagar

23. Handoff schemes based on algorithms of handoff (handoff protocols) Three strategies have been proposed to detect the need for handoff: • MCHO (mobile-controlled handoff) • NCHO (network-controlled handoff) • MAHO (mobile-assisted handoff) vidya sagar

24. Mobile‑Controlled Handoff (MCHO) • The MS continuously monitors the signals of the surrounding BSs and initiates the handoff process when some handoff criteria are met. • MCHO is used in DECT and PACS. • Digitally Enhanced Cordless Telecommunications system • Personal Access Communication System vidya sagar

25. Network‑ControlledHandoff (NCHO) • The surrounding BSs measure the signal from the MS, and the network initiates the handoff process when some handoff criteria are met. • NCHO is used in CT‑2 Plus and AMPS. vidya sagar

26. Mobile‑Assisted Handoff (MAHO) • The network asks the MS to measure the signal from the surrounding BSs. The network makes the handoff decision based on reports from the MS. • MAHO is used in GSM and IS‑95 CDMA. vidya sagar

27. INITIATION OF HANDOFF • Signal strength - reverse voice channel • Threshold level - minimum required voice quality • Cell site – MTSO • Unnecessary Handoff • Failure Handoff

29. The velocity of vehicle V and the pathloss slope γ , can be used to determine the value of Δ dynamically

30. When the mobile unit is located at a signal-strength hole within a cell but not at the boundary When the mobile unit approaches a cell boundary but no channels in the new cell are available. Two circumstances where handoffs are necessary but cannot be made

31. DELAYING A HANDOFF • A Two-Level Handoff Algorithm • Advantage of Delayed Handoffs • Switching processor • Interference

34. FORCED HANDOFFS A forced handoff is defined as a handoff that would normally occur but is prevented from happening, or a handoff that should not occur but is forced to happen.

35. QUEUING OF HANDOFFS • Queuing of handoffs is more effective than two-threshold-level handoffs • 1/μ - average calling time in seconds, including new calls and handoff calls in each cell • λ1 - arrival rate (λ1 calls per second) for originating calls • Λ2 - arrival rate (λ2 handoff calls per second) for handoff calls • M1- size of queue for originating calls • M2 -size of queue for handoff calls • N- number of voice channels • a =(λ1 + λ2)/μ • b1 =λ1/μ • b2= λ2/μ

36. Case – 1 • No queuing on either the originating calls or the handoff calls • The blocking for either an originating call or a handoff call is

37. Case-2 • Queuing the originating calls but not the handoff calls • The blocking probability for originating calls is • The blocking probability for handoff calls is

38. Case-3 Queuing the handoff calls but not the originating calls

39. POWER-DIFFERENCE HANDOFFS • power difference (∆)

40. Handover Performance Metrics • Cell blocking probability – probability of a new call being blocked • Call dropping probability – probability that a call is terminated due to a handover • Call completion probability – probability that an admitted call is not dropped before it terminates • Probability of unsuccessful handover – probability that a handover is executed while the reception conditions are inadequate vidya sagar

41. Handover Performance Metrics • Handoff blocking probability – probability that a handoff cannot be successfully completed • Handoff probability – probability that a handoff occurs before call termination • Rate of handoff – number of handoffs per unit time • Interruption duration – duration of time during a handoff in which a mobile is not connected to either base station • Handoff delay – distance the mobile moves from the point at which the handoff should occur to the point at which it does occur vidya sagar

42. Practical Handoff Consideration • Different type of users • High speed users need frequent handoff during a call. • Low speed users may never need a handoff during a call. • Micro cells to provide capacity, the MSC can become burdened if high speed users are constantly being passed between very small cells. • Minimize handoff intervention • handle the simultaneous traffic of high speed and low speed users. • Large and small cells can be located at a single location (umbrella cell) • different antenna height • different power level • Cell dragging problem: pedestrian users provide a very strong signal to the base station • The user may travel deep within a neighboring cell vidya sagar

43. Dropped Call Rates • The dropped call is defined as an established call which leaves the system before it is normally terminated • The Dropped Call Rate (DCR) parameter represents what percentage of all established calls is dropped during a specified time period • The DCR and voice quality are inversely proportional and high DCR may indicate coverage, handoff, or channels accessibility problems

44. The perception of dropped call rate by the subscribers can be higher due to: 1. The subscriber unit not functioning properly (needs repair). 2. The user operating the portable unit in a vehicle (misused). 3. The user not knowing how to get the best reception from a portable unit (needs education).

45. Relationship Among Capacity, Voice Quality, Dropped Call Rate Radio Capacity m is expressed as follows:

46. Formula of Dropped Call Rate • General Formula of Dropped Call Rate The general formula of dropped call rate P in a whole system can be expressed as: Where And

47. Thank you………………